Line integrals
Applets
Vector field microscope: Plots any vector field and allows you to see flows, divergence, and curls. [University of Arizona]
Line integrals in the plane: Plots any curve in 2 dimensions, and shows you the action of a given vector field on it. [Eric Carlen]
Line integral plotter: Plots any curve in 3 dimensions, and shows you the action of a given vector field on it. [MIT]